Effects of Porphyrin Core Saddling, meso-Phenyl Twisting, and Counterions on the Optical Properties of meso-Tetraphenylporphyrin Diacids: The [H4TPP](X)2 (X = F, Cl, Br, I) Series as a Case Study

The ground- and excited-state properties of a series of meso-tetraphenylporphyrin (H2TPP) diacids, [H4TPP](X)2 (X = F, Cl, Br, I), ad hoc synthesized and characterized by 1H NMR, RLS, and UV−vis spectroscopies, have been studied theoretically using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Several conformations corresponding to different deformations of the porphyrin core have been explored. The nearly degenerate purely saddled (sad) and hybrid (saddled with a small superimposed ruffling:  sadruf) conformations are the preferred “gas phase” conformations. The type and degree of distortion of the macrocycle and the orientation of the phenyl rings compare well to X-ray data available for H2TPP diacids. Two electronic structure features are key to an understanding of the optical and photophysical properties. (1) Strong interaction of the π-system of the phenyls with the π-system of the porphyrin leads to an upshift of the G-a2u (G = Gouterman) orbital and, hence, to a significant splitting of the occupied pair of a2u/a1u Gouterman orbitals. The diminished G-a2u/G-eg* gap and the lifting of the a2u/a1u degeneracy explain the red shift of the Q and B bands and the hyperchromicity of the Q-band in the diacids. (2) The highest occupied orbitals of the diacids comprise the set of halide lone pair orbitals, which move from completely above the Gouterman orbitals (I- counterion) to below them (F-). The lowest halide to porphyrin charge-transfer (HPCT) transitions are therefore predicted at very low energy (to the red of the Q-band) for Cl-−I-, but with very low intensity. Weak measured absorptions to the red of the Q-band support these theoretical findings. Quenching of the S1 (Q) state via these low-lying singlet HPCT excited states accounts for the decrease of the fluorescence quantum yield and for the measured trend along the series.